1. Field of the Invention
The present invention relates to an organic electroluminescent display device, and more particularly, to an active matrix organic electroluminescent display device including a thin film transistor and a fabricating method thereof.
2. Discussion of the Related Art
A cathode ray tube (CRT) has been commonly used as a display screen for devices such as televisions and computer monitors. However, a CRT has the disadvantages of being large, heavy, and requiring a high drive voltage. As a result, flat panel displays (FPDs) that are smaller, lighter, and require less power have grown in popularity. Liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are some of the types of FPDs that have been introduced in recent years.
An ELD device may either be an inorganic electroluminescent display device or an organic electroluminescent display (OELD) device depending upon the source material used to excite carriers in the device. OELD devices have been particularly popular because they have bright displays, low drive voltages, and can produce natural color images incorporating the entire visible light range. Additionally, OELD devices have a preferred contrast ratio because they are self-luminescent. OELD devices can easily display moving images because they have a short response time of only several microseconds. Moreover, such devices are not limited to a restricted viewing angle as other ELD devices are. OELD devices are stable at low temperatures. Furthermore, their driving circuits can be cheaply and easily fabricated because the circuits only require a low operating voltage. In addition, the manufacturing process of OELD devices is relatively simple.
In general, an OELD device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons with the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Since the mechanism by which an OELD produces light is similar to a light emitting diode (LED), the organic electroluminescent display device may also be called an organic light emitting diode.
In an organic electroluminescent display device, multiple organic electroluminescent layers may be used in which each layer emits red light, green light, or blue light in order to display full color images. Because any of the organic electroluminescent layers may break down over the course of time, it may be difficult to maintain the range of all possible colors when the organic electroluminescent display device has been driven for a long period of time. To solve this problem, a method of displaying full color images by using a single organic electroluminescent layer for all pixels and a color changing medium has been suggested in U.S. Pat. No. 5,294,870, which are hereby incorporated by reference. This method will be illustrated in FIGS. 1 to 3.
FIG. 1 is a plan view of an organic electroluminescent display device according to the related art. In FIG. 1, a planarization layer 101 is formed on a substrate, and a plurality of first electrodes R1–R5, which are spaced apart from each other, are formed on the planarization layer 101 along a first direction. An organic electroluminescent layer 8 is formed on the plurality of first electrodes R1–R5. The organic electroluminescent layer 8 is electrically connected to the plurality of first electrodes R1–R5. A plurality of second electrode portions C1–C6, which are spaced apart from each other, are formed on the organic electroluminescent layer 8 along a second direction that is substantially perpendicular to the first direction. Each second electrode portion C1–C6 includes three sub-electrodes “a,” “b” and “c.” The second electrode portion C1-C6 crosses the first electrode R1–R5, thereby defining pixel regions, of which a representative pixel region is “P.” The pixel region “P” includes three sub-pixel regions “Rp,” “Gp” and “Bp” of red, green and blue that are defined by the sub-electrodes “a,” “b” and “c” and the first electrodes R1–R5. External signals are applied through a peripheral portion “A” where the electroluminescent layer 8 is not formed.
FIG. 2 is a cross-sectional view of the organic electroluminescent display device of FIG. 1 taken along II—II according to the related art. FIG. 3 is a cross-sectional view of the organic electroluminescent display device of FIG. 1 taken along III—III according to the related art.
In FIGS. 2 and 3, green color changing medium “G” and red color changing medium “R” are formed on a substrate 2. The green and red color changing media “G” and “R” correspond to green and red sub-pixel regions “Gp” and “Rp,” respectively. The green and red color changing media “G” and “R” may be made of a material not susceptible to a photolithographic process. Next, a planarization layer 4 is formed on the green and red color changing media “G” and “R” to planarize a surface of the substrate 2 and separate adjacent green and red sub-pixel regions “Gp” and “Rp.” The planarization layer 4 is made of transparent insulating material through a spin coating method or a solgel method without an additional patterning process. The planarization layer 4 also protects the green and red color changing media “R” and “G.” Next, a plurality of first electrodes “R1” and “R3” are formed on the planarization layer 4. The plurality of first electrodes “R1” and “R3” are made of transparent conductive material such as indium-tinoxide (ITO) to transmit light.
Next, a sidewall 6 is formed on the plurality of first electrodes “R1” and “R3” at a boundary of the green and red sub-pixel regions “Gp” and “Rp.” The sidewall 6 may be formed through depositing and patterning photoresist. The sidewall 6 may be made of silicon oxide (SiO2), silicon nitride (SiNx) or aluminum oxide (Al2O3). Next, an organic electroluminescent layer 8 is formed on the sidewall 6 and the plurality of first electrodes “R1” and “R3.” The organic electroluminescent layer 8 is made of a material emitting blue light. A plurality of sub-electrodes “a,” “b” and “c,” which function in combination as a second electrode, are formed on the organic electroluminescent layer 8. Preferably, the plurality of sub-electrodes “a,” “b” and “c” are made of a material having a low work function so that each is substantially efficient for proper operation of the electroluminescent display device. When the plurality of sub-electrodes “a,” “b” and “c” are formed through a sputtering method, the positioning of a target including a source material is important in order that the plurality of sub-electrodes “a,” “b” and “c” are properly spaced. If the target is close to a first surface “X” of the sidewall 6, the source material is deposited on the first surface “X” of the sidewall 6, but the source material is not deposited on a second surface “Y” of the sidewall 6 and a portion of the organic electroluminescent layer 8 adjacent to the second surface “Y.” Accordingly, the organic electroluminescent layer 8 has a gap between the adjacent sub-electrodes “a,” “b” and “c” along a first direction.
The organic electroluminescent display device of FIGS. 1 to 3 is a passive matrix organic electroluminescent display device. In the passive matrix organic electroluminescent display device, scan lines are sequentially driven so that the brightness of each pixel may be appropriately determined. Accordingly, the brightness for which a pixel is driven should be the multiple of the desired average brightness and the number of scan lines required to obtain the desired average brightness. Thus, as the number of scan lines increases, the required supply voltage and supply current increase as well. An increase in the required supply voltage and supply current accelerates the degradation of a device and increases the power consumption of the device. Although a passive matrix organic electroluminescent display device may be adequate for small display devices, it is not an adequate solution in larger display devices.